Built different: ER cisternae formed by the Arabidopsis Lunapark proteins differ in ultrastructure and affect ER-Golgi transport.

The plant endoplasmic reticulum (ER) is a dynamic organelle composed of multiple distinct structural domains, such as cisternae, which are maintained by ER morphogens including the Arabidopsis thaliana Lunapark proteins (LNPs). Cisternae are typically described as sac-like structures connected by tubules. Here we challenge this assumption and propose that cisternae have a more complex structure that modifies ER functionality. This study used state-of-the-art high-resolution confocal and variable-angle epifluorescence microscopy, along with transmission electron microscopy and tomography, on high-pressure frozen Arabidopsis thaliana samples. We found that AtLNP1-stabilised ER forms cisternae composed of dense tubular matrices, whereas AtLNP2 forms cisternae with a uniform, sac-like structure. Furthermore, overexpression of AtLNP proteins alters Golgi morphology, affecting ER-to-Golgi transport and secretion. Our findings reveal that the balance between AtLNP1 and AtLNP2 is critical for ER cisternae organisation and ER functionality in protein production and secretion. This work provides new insights into ER structural plasticity and its functional implications in plant cells.